I'd like to refresh the compiler flags configured by sbt-typelevel which is used to cross-compile several libraries for both Scala 2 and 3. I have a question about:

• -Xsource:3 in Scala 2, which "enables some Scala 3 syntax and behavior"

• -source:3.0-migration in Scala 3, which "accepts some of the old Scala 2.13 syntax"

Ref: https://docs.scala-lang.org/scala3/guides/migration/tooling-tour.html

Does it make sense to use both of these flags in a cross-compiled codebase? I ask, because the former sounds like "make Scala 2 accept Scala 3-ish sources" and the latter sounds like "make Scala 3 accept Scala 2-ish sources". So I wonder if we should pick one strategy (aim for either Scala 2-ish or Scala 3-ish sources) and use the one flag that makes sense for that.

Also what about -source:3.3-migration? 😅 Can that be mixed with -Xsource:3?

Case Study

Consider the following program:

case class Foo private (bar: String)
object Main {
 def main(args: Array[String]): Unit =
 println(Foo("bar"))
}

Actually, this doesn't compile out-of-the-box with Scala 3.3.0.

[error] object Foo does not take parameters
[error] println(Foo("bar"))
[error] ^^^

Now here is what happens with various Scala version and compiler flag configurations:

Analysis:

• -Xsource:3 and -source:3.0-migration have the opposite effect on their respective compilers. So using them together seems pretty dumb.
• -Xsource:3 and -source:3.3-migration in this specific situation have the same result. So maybe they can be used together?